The present invention relates to a system for optical information storage, and more particularly to an erasable system for optical information storage in which information is read from a magneto-optical recording medium by employing a semiconductor laser as a light source.
An erasable apparatus for optical information storage may be in the form of a magneto-optical disk memory which uses a perpendicularly magnetizable film as a storage medium. In such an apparatus, the storage medium is irradiated with a focused laser beam so as to invert the direction of magnetization thereof by absorbing the photon energy of the laser beam so as to record and erase information with the rotation of the polarization plane of light reflected from the storage medium being detected so as to reproduce information (Y. TOGAMI et al, "Amorphous thin film disk for magneto-optical memory", SPIE, Vol. 329, Optical Disk Technology (1982). pp. 208-214). With the magneto-optical storage apparatus of this type, the rotation power of the magnetizable film for the polarization plane is very low, for example, below 1.degree. in terms of an angle. This leads to the serious problem of the S/N (signal-to-noise) ratio of a reproduced signal being too small for realization of a high-speed readout.
On the other hand, an optical disk memory having a good S/N ratio may be in the form of an optical disk memory of the type which is used only for reproduction or the type which is capable of recording and reproduction, but is incapable of erasure and rerecording. The reproduction in the optical disk memory of this type is based on detecting the magnitude of the intensity of reflected light from the disk. When employing a semiconductor laser as a light source, it is also possible to reproduce information by exploiting the so-called self-coupling effect which is a phenomenon wherein, when the reflected light from the disk is returned to the semiconductor laser, the optical output of the laser changes depending upon the magnitude of the intensity of the reflected light. Moreover, the use of the semiconductor laser as the light source is very advantageous for rendering the apparatus small in size and light in weight as well as in lowering the cost thereof.
An example of such an information reproducing apparatus which utilizes the self-coupling effect of the semiconductor laser is disclosed in Japanese Laid-open Patent Application No. 51-126846. In this apparatus, the magnitude of the intensity of the reflected light from the disk is converted into the magnitude of a backward emergent light of the semiconductor laser. Therefore, information is reproduced by a photodetector which is disposed behind the laser. In this apparatus, the polarization of the forward emergent light of the laser onto the disk is a TE (Transverse Electric) polarization which is parallel to the plane of a p-n junction, and also the polarization of the reflected light from the disk, as well as the backward emergent light of the laser is a TE polarization. Such an apparatus, however, merely operates to read recorded information and cannot operate to erase the information and record new information.
Recently apparatuses have been proposed which read information from an erasable storage medium in the form of a magneto-optic recording medium with utilization of the self-coupling effect. However, the rotation of a polarization plane by the magnetization of the magnetic film is slight, and a satisfactory S/N ratio of a reproduced signal has not been attained.
For example, Japanese Laid-open Patent Application No. 56-94530 discloses a reproducing apparatus wherein a double refractive material such as .lambda./4 plate is disposed in an optical path along which reflected light from the magneto-optic storage medium returns to a semiconductor laser, and wherein an angle defined between the optical axis of the .lambda./4 plate and the polarization plane of irradiating laser light is selected so that the oscillation mode of the semiconductor laser is changeable between a single oscillation mode and a multiple oscillation mode, depending upon the state of the magnetization of the storage medium.
In this regard, however, whether the semiconductor laser fed back with the reflected light effects the single mode oscillation or the multi-mode oscillation depends upon the quantity of the reflected light, the distance from the laser to the point of reflection, the temperature of the laser and the driving current of the laser, and it is particularly susceptible to changes in the distance between the laser and the medium. Accordingly, it is virtually impossible to detect the sense of the magnetization on the basis of the change in the oscillation modes.
In addition, Japanese Laid-open Patent Application No. 56-16950 discloses a reproducing apparatus wherein a Faraday rotation device is disposed in an optical path along which reflected light from a magneto-optic storage medium returns to a semiconductor laser. This apparatus exploits the phenomenon that the optical output intensity of the semiconductor laser changes sinusoidally, depending upon the rotation of the polarization plane of the reflected light fed back to the laser. In this apparatus, like the aforedescribed apparatus of Japanese Laid-open Patent Application No. 51-126846, the polarization of the forward emergent light of the laser (irradiating beam) is the TE polarization, and also the polarization of the reflected light from the storage medium, as well as the backward emergent light of the laser (reproducing beam) is the TE polarization. That is, the oscillating polarization direction of the semiconductor laser does not change.
Moreover, since the rotational angle of the polarization plane by the magneto-optical storage medium is as small as less than 1.degree., that variation of the optical output of the laser which corresponds to the change of the sense of the magnetization of the backward emergent light of the laser is slight. Accordingly, the S/N ratio of a reproduced signal is very low.